Casting process for nickel base alloys

ABSTRACT

A casting process and castings produced thereby wherein a nickel-base, chrominum containing alloy which also contains sufficient amounts of aluminum and titanium to produce eutectic gamma-prime phase in the alloy as cast is caused to contain about 0.5 percent to about 5 percent hafnium, is cast into final configuration and is cooled from the molten state rapidly through the range of 2,200* to 1,850* F. to maximize creep characteristics at 1,800* F. while at the same time providing castings which are thermally conditioned to exhibit good mechanical characteristics, especially prior creep at 1,400* F.

United States Patent Lund et a]. [45] July 18,1972

[54] CASTING PROCESS FOR NICKEL BASE 3,260,505 7/ 1966 Ver Snyder ..4l6/241 X LL 3,312,449 4/1967 Chandley.... 16/241 A OYS 3,494,709 2/1970 Piearcey..... 416/232 [72] inventors: Carl H. Lund, Arlington Heights; John 3,005,705 10/1961 Cochardt.... ..75/ l 71 Hockin, Palatine; Michael J. Woulds, 3,526,499 9/1970 Quigg et a1... ..75/171 Schaumburg, all of 111. 3,552,479 1/ 1971 Hockin ..164/ 122 X [73] Assignee: Martin Marietta Corporation, New York, Pn-mmy Examiner j Spencer overhok.)er

- N .Y. Amman! Examiner-l0hn E. Roethel [22] Filed: July 14 1969 Attorney-John A. Crowley, Jr. and Francis J. Mulligan, Jr.

[2]] Appl. No.: 841,408 [57] ABSTRACT m s Application n A casting process and castings produced thereby wherein a nickel-base, chrominum containing alloy which also contains comlnuailon-ln-pan of 725,074, P" 29, sufficient amounts of aluminum and titanium to produce eu- 1968 abafldonedtectic gamma-prime phase in the alloy as cast is caused to contain about 0.5 percent to about 5 percent hafnium, is cast into [52] U.S. Cl. 164/122, 75/171 final configuration and is cooled from the molten state rapidly [51] Int. Cl ..B22d 27/04 through the range of 2,200 to 1,850 F. to maximize creep [58] Field of Search ..416/223, 241; 29/187; 164/55, characteristics at 1,800 F. while at the same time providing 164/57, 58, 122; 75/ 171 castings which are thermally conditioned to exhibit good mechanical characteristics, especially prior creep at 1,400 F. [56] References Cited 5 Claim, 16 Drawing Figures UNITED STATES PATENTS 3,129,069 4/1964 Hanink et a1 ..4l6/24l X v. PRIOR cam AT I400'F RELATIVE COOLING RATE THROUGH 2400F- 2200F Patented July 18, 1972 3,677,331

4 Sheets-Sheet l F/G. IA PRIOR CREEP PRIOR ART AT T400E L I5 20 5 RELATIVE COOLING RATE THROUGH 24o0F-22o0 E STRESS RuPTgRE LIFE AT I800 F WITH A LOAD ZS Z 0F 29 KSI rT:'.": I 2 a 4 5 6 T a 9 .10 II RELATIVE Rcoouuc RATE THROUGH 2200 F- I850 F P76. /0 PRIOR cREEP AT |400F T T 5 l0 I5 20 25 RELATTvE coouuc RATE TRRoucR 2400E- 2200F STRESS RUPTURE LIFE AT |800F WITH A LOAD 0F 29 KSI FIG. /0 r f::::.,=1

l 2 3 4 5 6 l 8 9 IXT'LEINTORS 0 0 RELATIVE COOLING RATE THROUGH 2200 F- I850 F CARL H LUND JOHN HOCKIN MICHAEL J. VIOULOS MTfiORXEF Patented July 18, 1972 4 Sheets-Sheet 2 Fl G 2 PRIOR ART Patented July is, 1972 3,677,331

4 Sheets-Sheet 5.

FIG.6 FIG.7

FIG.8

. P IOR ART PRIOR ART 4 Sheets-Sheet 4 FIG.|O FIG.

PRIOR ART CASTING PROCESS FOR NICKEL BASE ALLOYS The present application is a continuation-in-part of US. application Ser. No. 725,074, filed on Apr. 29, 1968, now abandoned, in the names of Carl H. Lund, John Hockin and Michael J. Woulds.

The present invention is concerned with a process of casting and, more particularly, with a process of casting and the cast products produced thereby.

It is known that when molten alloy is poured into a mold and allowed to solidify therein, the homogeneity which usually characterizes the liquid state is lost in the solid state to one degree or another. Even if one ignores such obvious items as inclusion of contaminants introduced during casting, pores, cold shuts, pipes and the like and presumes the ultimate in casting technique, cast alloy is still characterized by macro and micro segregation which is the inherent result of the freezing process. If the casting is a billet (or other similar shape) which is to be plastically worked and thereby homogenized, the segregations induced by the freezing process are not overly important since homogenization during working will tend to minimize freezing induced segregation. On the other hand, if the cast object is to be used in its cast configuration, that is, without being plastically worked after solidification, segregation due to freezing is very important because only thermal homogenization techniques can be used to reduce segregation.

ln alloys of the kind employed to produce castings useful in hot-stage, gas turbine engine service, freezing-induced segregation is a severe problem. Generally speaking, such alloys contain a major amount of nickel, much smaller amounts of chromium, cobalt, aluminum and titanium, small amounts of one or more of molybdenum, tungsten, columbium, vanadium, tantalum, till much smaller amounts of carbon and zirconium and usually a very small amount of boron. Physically, the frozen alloy comprises a matrix phase, one or more carbide phases dispersed in said matrix phase a precipitate of gamma-prime and an intermetallic eutectic (Ni Al Ti) phase called eutectic gamma-prime phase also dispersed throughout the matrix in sufficient amount such that it forms directly on solidification of the alloy or in the temperature range immediately below freezing and is not readily fully solutionable in the solid matrix. From the foregoing description of the castings useful in hot-stage, gas turbine engine service, it will be appreciated that freezing induced segregation can take many different forms. For example, different parts of the matrix can have different compositions. Carbides can gather at preferred matrix locations, eg at the grain boundaries. The particles of gamma-prime phase can differ in composition one from another and at one location is a given particle from another location in the same particle. Accordingly, although the alloys of the kind in question are inherently ofa segregated nature by virtue of their contained phases, additional segregation at freezing or in the temperature interval therebelow down to about l,850 F. can and does often occur. This segregation can be highly deleterious in that it can provide metallurgically unstable areas in the castings and further can cause or contribute to the occurrence of overly weak and overly brittle areas in the castings.

It has been found in castings useful in hot-stage gas turbine service that the mechanical characteristics of the castings at various temperatures are highly dependent on the cooling rates at two contiguous temperature intervals. To obtain the optimum high temperature (about l,800 F.) mechanical characteristics it is desirable that the casting cool rapidly through the range of 2,200 F, to 1,800 F. To obtain the optimum 1,400 F. mechanical characteristics, it is desirable that the casting cool relatively slowly through the temperature range of 2,400 F. to 2,200 F. It is obvious that without highly sophisticated and expensive apparatus, these cooling desiderata are very difficult to attain.

A further complicating factor in any endeavor to minimize the deleterious effects of unsuitable cooling rates is the fact that castings suitable for hot stage gas turbine engine usage are not usually of simple configuration. When a test bar comprising a cylindrical or tapered cylindrical portion adapted to be exposed to the test conditions is cast in a precision shell mold, the round cross section normal to its longitudinal axis tends to assure that the heat gradient in any selected radial direction will be essentially equal to the heat gradient in any other radial direction during and after solidification On the other hand, with a turbine blade casting wherein the cross section in the blade area normal to the longitudinal axis is highly non-symmetrical with respect to any possible point of symmetry the heat gradients in various directions across the section will be substantially different. Accordingly, it is clear that, as a practical matter, castings suitable for use in hot stages of gas turbines cannot be made under conditions which will maximize mechanical characteristics at both l,400 F. and 1.800 F. at all points in the casting. It is readily possible, however, to cast and cool under conditions which tend to maximize mechanical characteristics at 1,800 F. In prior art processes the result of such a procedure is to produce castings which quite often do not measure up to specifications relating to percent prior creep at l,400 F.

lt is a discovery of the present invention that, by means of a new casting process, novel cast objects can be produced having excellent mechanical characteristics both at l,800 F. and 1 ,400 F.

It is an object of the present invention to provide a novel casting process.

Another object of the present invention is to provide novel, highly useful cast objects by means of said novel casting process.

Other objects and advantages will become apparent from the following description taken in conjunction with the drawings in which:

FIG. 1 is a series of graphs interrelating mechanical characteristics and relative cooling rates of castings through selected temperature ranges; and

FIGS. 2 to 13 comprise a series of reproductions of photomicrographs from various loci in sections of turbine blade castings.

Generally speaking, the present invention contemplates a process wherein an alloy, having a nickel base and containing chromium and having in the solidified state a gamma matrix phase, a carbide phase dispersed in said matrix a gamma prime precipitate and an essentially non-solutionable, eutectic gamma-prime phase dispersed in said matrix, is caused to contain about 0.5 to about 5 percent by weight of hafnium; is cast from the molten state into a mold having essentially the final configuration of use of the object thus cast; and is caused to solidify in said mold at a rate tending to maximize mechanical characteristics as measured at l,800 F. Advantageously, the melting, casting and solidification operations are carried out under high vacuum. Further, in order to maximize those conditions which tend to insure in the resultant casting good mechanical characteristics at 1,800 E, it is advantageous to cool the casting rapidly through the range of 2,200 F. to 1,850 F. The process of the present invention, comprising a combination of processing operations and inclusion of critical amounts of hafnium on nickelbase alloys of a specified type, results in castings which not only have excellent mechanical characteristics at l,800 F. but also at the same time have excellent mechanical characteristics at intermediate temperatures of about 1,400 F.

There are certain practical difficulties in demonstrating the advantages of the process of the present invention as applied to casting hot stage, gas turbine structures because many of such structures are of complex airfoil shaped cross section and are not adapted to provide more than one test specimen per casting. Thus it is difficult, if not impossible, to prove directly on cast hardware the efficacy of the invention. Consequently, indirect evidence based upon cast-to-size alloy test specimens cooled at various rates through the ranges of 2,400 to 2,200 F. and 2,200 to 1,800" F. is given to show the effects of the invention. It is important to note, however, that the mechanical characteristics exhibited by cast-to-size test specimens are not representative of the mechanical characteristics of items of cast hotbstage, gas turbine hardware. Rather, such cast-to-size specimens represent the mechanical characteristics of only that portion of the cast item which is subject to the same cooling treatment as the specimen. An actual cast item would exhibit a range of characteristics dependent upon the location from which a test specimen might be taken. Thus one of the important factors to be considered in evaluating the test data set forth herein is the relative sizeof the ranges of cooling rates which can be employed to give acceptable mechanical characteristics in castings. It is a major advantage of the process of the present invention that relatively wide ranges of cooling rates will produce acceptable mechanical characteristics at both l,800 F. and l,400 F. Thus, castings produced by the process of the present invention will contain much greater amounts of alloy thermally conditioned to give good mechanical characteristics at both l,800 and l,400 F. than are present in castings produced by prior art processes.

A comparison of the process of the present invention and the processes of the prior art is shown in FIG. 1 of the drawing. In this figure are set forth four graphs, the upper pair of which represent the prior art and the lower pair of which represent the present invention. The data on which these graphs is based was obtained from a series of cast-to-size test bars made at various cooling rates spanning the range of cooling rates normally encountered in casting airfoil-shaped hot stage, gas turbine blades. Test bars representing relative cooling rates were tested for lift-to-rupture at 1,800" F. under a load of 29 k.s.i. and for percent prior creep(percentage of creep more than one hour prior to rupture) at 1,400" F. under a load of 94 lt.s.i. The alloys employed to give data for both pairs of graphs were identical nickel-base alloys containing eutectic gamma-prime phase as cast except that, in accordance with the requirements of the present invention, the alloy employed to obtain the data for the lower graph was caused to contain about 1.5 percent hafnium.

Graphs B and D of FIG. 1 show that the stress-rupture lives at 1,800 F. of metal in castings is essentially unaffected by use of the precess of the present invention. When the cooling rate is slow through the range of 2,200 to l,850 F. the lift-to-rupture of cast metal at 1,800 F. under high stress is relatively poor. When the cooling rateis fast through this range, the lifeto-rupture is relatively good. The minimum of an acceptable range of cooling rates is fixed by engineering judgment as to the minimum acceptable life-to-rupture at l,800 F. and is the same in both graphs. Essentially, thecurve is the same in both of graphs B and D except that the curve of graph D (derived from alloy specimens containing hafnium) is drawn more diffuse to indicate the possibility that the process of the present invention may narrow slightly the range of cooling rates which give good stress-rupture characteristics at l,800 F. This slight narrowing is of no real practical significance since practically obtainable cooling rates indicated by the cross hatched boxes in FIG. 1 are safely within the acceptable range of cooling rates. The curve representing percent prior creep is another matter. In the instance of graph A (based on alloys devoid of hafnium), the curve representing percent prior creep shows a high percent priorcreep at a slow cooling rate through the range of 2,200 to 1,850 F. At very fast cooling rates the percent prior creep is very low and is unacceptable. The maximum of the acceptable range of cooling rates is fixed by engineering judgement as to the minimum acceptable percent prior creep. In graph C (derived from alloy specimens containing hafnium), the curve representing percent prior creep shows not only a higher value at very, slow cooling rates but also a difference in slope such that at very fast cooling rates the percent prior creep is still above an acceptable level. The cross hatched boxes on graphs A and C show the range of cooling rates normally encountered in the cooling of gas turbine blade castings from 2,400 to 2,200 F. In the process of the invention this range of practical cooling rates is entirely within the acceptable range of cooling rates whereas, in prior art processes, the acceptable range of cooling rates barely abuts on the range of cooling rates encountered in practice. The result of these phenomena, directly applicable to castings produced in accordance with the process of the present invention, is a large proportion of metal in, for example, a turbine blade casting, which has been subjected to a thermal history adapted to provide acceptable mechanical characteristics both at l,800 F. and l,400 F. In contrast, in the prior art processes, only that small amount of metal having a thermal history within the narrow acceptable ranges of cooling rates can be considered to have acceptable mechanical characteristics both at l,800 F. and l,400 F. It is to be noted that the graphs of FIG. 1, although based upon experimentally obtained and mathematically derived data, are diagrammatic in nature rather than precise and exact. The relative cooling rates have been expressed in dimensionless units which have been obtained from theoretically calculated, numerically identical, rates of cooling in degrees Fahrenheit per second. It is the intent of graphs A to D of FIG. 1 to show trends based on relative cooling rates rather than a prediction of specific results to be obtained for specific mechanical characteristics with specific cooling rates. For thisreason specific values of mechanical characteristics have not been used, but rather only linear scales have been indicated. Another factor which tends to prohibit use of FIG. I as a precise tool is the probability that, at any increment of time during the cooling of a casting, every point within the casting will be cooling at a different rate from almost any other point. In calculating rates of cooling, one-half of a typical turbine blade section was divided into some 45 nodes with an additional 138 nodes being employed to characterize the surrounding refractory and boundaries. What was calculated and indicated in FIG. 1 as the range of cooling rates was the range of average rates of cooling through the two temperature ranges of three nodes representing metal from the leading edge, the trailing edge and mid-chord of the turbine blade. These average rates are set forth in Table I.

TABLE I Temperature Cooling Rates Range Trailing Edge Mid-Chord Leading Edge 2400-2200F. 24F./s

econd 7 25F .lsecond l2F./second 2200-l 850F. 5. 8F./

second 7F./second 6.9F./second Those skilled in the art will appreciate that the tabulated rates are themselves average of rates over the range of temperature and further represent averages of rates of cooling of metal within each of the three-selected nodes. Accordingly, while the trends suggested by FIG. 1 are accurate and realistic as trends, FIG. 1 cannot be used to specify precisely the characteristics of any specified volume of metal in a casting.

The castings of the present invention are characterized by substantial uniformity of basic metallographic structure across sections thereof even such sections as those cut transversely across an airfoil shape. In this regard, basic metallographic characteristics are intended to include the relationship of carbides and gamma-prime phase to the grain structure. The particular form in which the gamma-prime phase occurs within the grain structure is not particularly uniform across a transverse blade section but this species of non-uniformity does not appear to be detrimental. FIGS. 2 to 13 are photomicrographs in sets as tabulated in Table II.

8 trailing edge B No 9 center B No 10 leading edge B No 11 trailing edge B+l.5% Hf Yes 12 center B+l.5% Hf Yes 13 leading edge B+l.5% Hf Yes The composition of alloys A and B are set forth hereinafter.

In each instance of a casting of the invention, as shown in the drawing, the basic grain structure of the cast alloy is not readily visible. This is because carbides, which in prior art castings form preferably at the grain boundaries, are scattered throughout the grain structure in the castings of the present invention and thus do not outline the grain boundaries. Furthermore, gamma-prime envelopes which can form at grain boundaries in prior castings are essentially absent in castings of the present invention. The absence of gamma prime envelopes surrounding discrete and elongated carbides is important because such envelopes tend to provide areas in the casting which are sensitive to shear at intermediate temperatures of about l,400 F. especially after the casting is exposed to conditions of time at elevated temperature sufficient to induce diffusion of carbon within the casting.

From the product aspect, the present invention comprises cast high temperature turbine engine hardware, such as turbine blades, vanes, integrally cast turbine wheels and nozzle guide vanes and, in general, castings having a plurality of loci adapted to cool after solidification at different rates through both of the temperature ranges of 2,400 to 2,200 F. and 2,200 to 1,850 F. made from a nickel-base, chromium-containing alloy having, in its original form, commercially acceptable strength and ductility characteristics, which alloy is modified by the inclusion of at least 0.5 percent to about 5 percent (by weight) of hafnium in the cast alloy composition. As employed in the present invention, hafnium can be included in the cast alloy as a substitute for an equal percent by weight of nickel, tantalum and, at times, other refractory elements. Advantageously, hafnium, in amounts of about 0.6 percent to about 1.8 percent, is substituted for equal percents by weight of tantalum when tantalum is present in the basic alloy being improved in accordance with the present invention. Generally speaking, when hafnium is substituted for elements such as tantalum in an alloy, it is likely that the strength characteristics of the basic alloy will not be significantly changed while, at the same time, the ductility of the alloy will be significantly enhanced. When hafnium is substituted for alloying elements such as nickel, the ductility characteristics of the alloy will be significantly improved while, at the same time, it is possible that the strength characteristics of the base alloy will be improved. It is to be observed, however, that the inclusion of hafnium in a nickel-base alloy already extremely loaded with hardening elements and designed specifically for the highest strength levels at the highest temperatures without due regard for ductility will not necessarily raise the ductility to levels of generally acceptable commercial standards at trough temperatures. In other words, the present invention is not a substitute for the exercise of reasonable metallurgical judgement in the balancing of alloy compositions to obtain a commercially viable compromise between strength and ductilit Iii the present invention, ductility deficiencies appear to be much more pronounced in castings made using revert, that is, previously remelted and cast alloy. lf test specimens, machined from turbine blades made from revert-containing heats are tested under creep inducing conditions at l,400 F it is possible that such tests will indicate the existence of practically no ductility if the casting is devoid of hafnium. On the other hand, if revert heats are modified to contain about 0.5 percent to about 2 percent hafnium, in accordance with the present invention, applicants have found that the revert-containing heats will provide hardware of a quality at least fully equivalent to the quality of hardware made from virgin heats.

Applicants do not fully understand the factors involved in the aspect of the present invention which is concerned with improvement in the character of hardware produced from revert heats. It is though, however, that use of revert inadvertently introduces certain types of undesirable impurities into the metal in very small, analytically undetectable amounts. Apparently hafnium has the ability to either combine with and effectively scavenge such impurities or to modify the metallurgical balance of the alloys in such a fashion so as to make the alloy less sensitive to the impurities. Regardless of the mechanism by which it operates, applicants invention is, in part, the discovery that by treating revert heats to include small quantities of hafnium therein, the characteristics of castings made from such revert heats can be made to substantially equal, if not better, the characteristics of castings made from virgin heats of substantially the same alloy.

The chemistry in weight percent of some known nickel-base alloys to which the present invention is applicable are set forth in Table 111.

It is to be observed that compositions set forth in Table 111 are nominal compositions and the percentage of each element present may be varied plus or minus about 10 percent of the amount specified. The alloys can also contain up to about 2 percent (by weight) total of incidental elements such as manganese, silicon, iron, etc. Non-metallics such as sulfur, oxygen and nitrogen and deleterious metallics such as lead, bismuth, arsenic, etc. are kept at as low a level possible consistent with good commercial practice. Advantageously, all of the alloys in Table 111 and the alloys of the present invention are prepared by vacuum melting and casting, while under vacuum, into investment-casting molds having gas turbine engine hardware form.

From Table 111 one skilled in the art will observe that castings made in accordance with the present invention to provide advantageous, hot stage, gas turbine engine hardware are made of alloys which fall within the range of composition, in weight percent, as set forth in Table IV.

Addition of hafnium, or, advantageously, substitution of hafnium in amounts of about 0.5 percent to about 1.5 percent or even 5 percent by weight in balanced alloy compositions within the range set forth in Table IV enhances ductility of the castings particularly in the trough region about 1,400 F. without detrimentally affecting other important engineering characteristics of the alloys. The hafnium addition is made to the alloy after deoxidation with no difficulty and, thereafter, the modified alloy is treated exactly as it would be if no modification has been made. In the case of revert heats, which are made with revert already containing hafnium, itis sometimes not necessary to add additional hafnium unless the aggregate composition of the heat or the processing technique lowers thehafnium content to below about 0.5 percent. Alloys within the range set forth in Table IV can be balanced especially with regard to the elements tantalum and tungsten (when tungsten is present) by maintaining the tungsten content in excess of about 8 percent by weight and maintaining the total of tantalum plus tungsten below about 13 percent or even percent by weight. Also, advantageously, the alloys within the range set forth in Table IV are balanced such thatv the carbon content is, at most, approximately equal to the hafnium content in an atom-for-atom basis.

. The present invention is more particularly concerned with castings having compositions as set forth (in weight percent) in Table V.

Alloys 1 and 2 set forth in Table V were melted and cast under vacuum to provide cast-to-size hardware and incidental specimens for chemical analysis. Comparative specimens were prepared from alloy series A and B which, except for hafnium, are essentially similar to Alloys 1 and 2, respectively.

The ductility characteristics of castings made of revert-containing heats of Alloy B and similar alloys of the present invention containing hafnium are shown in Tables V1 and VII. The data in Tables VI and VII were obtained ontest samples machined from cast turbine blades. The cast turbine blades were produced by casting alloy under vacuum into investment molds and causing said alloy to solidify in said molds under conditions tending to maximize mechanical characteristics at 1,800 F. Table VI contains room temperature tensile data. Table VII contains creep rupture data obtained at 1,400 F. under a load of 85k.s.i.

Table VI shows that inclusin of hafnium in the composition of Alloy B clearly improves room temperature tensile elongation exhibited by specimens machined from blades made from revert-containing heats.

TABLE VII Casting identification Hf Life-to-Rupture Prior Creep base alloy (nominal) (hours) Alloy B 0 13.4 0.42 Alloy B 0.5 177.2 3.98 AlloyB+1 131.8 2.18 Alloy B +1.5 136.9 2.91

The data in Table VII, especially the data as to Alloy B, shows the loss of ductility at 1,400 F. attendant on the use of revertcontaining heats in cast turbine blades- Experience has indicated generally that revert-containing heats of hafnium-containing castings (0.5 percent hafnium by weight) of the present invention are substantially equivalent in merit to castings made from virgin heats of essentially the same alloy without hafnium.

Table VIII sets forth the chemical compositions of some a1- loys from which castings of the present invention are made.

TABLE VIII Alloy l 2 3 4 5 6 7 8 9 Al 5.48 5.70 5.60583 6.03 6.13 6.06 6.15 B .017 .017 .0170.015 0.018 0.017 0.018 0.015 0.016 C .12 .14 .14 0.09 0.10 0.10 0.05 0.07 0.07 Cr 8.78 8.60 8.557.78 7.82 7.73 8.05 7.95 7.98 Co 10.1 10.0 9.859.88 9.80 9.78 9.79 9.79 9.72 Mo 2.52 2.47 2.436.00 5.77 5.75 6.05 6.10 6.12 NiB B B B B B B B B Ti 1.50 1.54 1.591.08 1.06 1.05 1.03 1.08 1.08 W 9.90 9.80 9.55 0.1 0.1 0.1 Zr.12 .11 .14 0.09 0.07 0.13 0.13 0.15 0.16 Hf 1.50 2.20 3.300.49 1.10 1.40 0.53 1.03 1.55 Ta 4.40 4.27 4.32 3.88 3.25 2.73

nium, hafnium master alloy or possibly by means of intermetallic hafnium compounds. Unless already contained in metal being melted, the hafnium should be included in molten alloy at a time after the melt has been refined, as by a carbon boil, and essentially killed to avoid formation of excessive amounts of the very stable hafnium oxide. As is well known to those skilled in the art, it is highly advantageous to melt and cast alloys, of the kind considered herein, under high vacuum in order to avoid inclusion of deleterious amounts of oxygen, nitrogen, etc. in the alloy. Under certain conditions, however, alloys considered herein can be melted under inert gas or other protective blankets and cast in air provided due care is taken.

Compositions of other alloys from which advantageous castings of the invention are made are set forth in terms of percent by weight of Table IX.

TABLEIX AI- loy 1o 11 12 1314 15 16 17 1s 19 c 0.15 010013010 0.120.18 0.050.120.140.0s0.12 Cr 9.0 9.0 9.0 9.0100100 12.09 9 9 12.5 C0 10.0 10.010.010.0 15.0 9 9 9 Mo 1.5 4.5 2.5 2.4 4.2 w 10.0 12.211.511.21 2.0 9.9 9.6 10 Ti 15 2.0 2.0 2.01.0 4.7 0.6 1.5 1.5 1.5 0.8 A 5.0 5.0 5.06.5 5.5 5.9 5.5 5.5 5.5 6.1 B 0.015 0.010.010.01 0.0200.0140.010- Zr 0.05 0.05 0.05 0.05 0.10006 0.10v Ta- 1.5 -2.0 1.5 Hf 2.5 1.6 1.1 2.3 2.5 2.5 1.5 1.5 3.3 4.5 1.5 Ni bal bal ba1.ba1 bal bal bal bal bal bal bal 5* 15* E 15* Includes small conventional amounts of boron and zirconium Additional evidence as to how the problem of low ductility at about l,400 F. which manifests itself in specimens machined from cast hardware is solved by the inclusion of more than about 0.7 percent hafnium, e.g. about 1 percent to about 4 percent hafnium, in virgin alloys is set forth in Tables X and X1. Table X contains creep data with respect to castings of heats of Alloy B which have been modified by inclusion of the various indicated amounts of hafnium therein and tested at l,400 F. The data in Table X was obtained on specimens machined from investment cast turbine blades cooled under conditions tending to maximize mechanical characteristics at TABLE X 1400F./94,000 psi Heat No. Wt. Hf Hours Prior Creep Specimens of castings made from virgin Alloy B, containing no hafnium which were prepared in the same manner as the specimens on which Table X is based, were tested under two A comparison of tables X and X1 shows that the problem in castings of unmodified Alloy B at l,400'F. manifesting itself as low life-to-rupture and low percent prior creep, as measured on specimens machined from blades, is completely overcome by inclusion of greater than 1 percent hafnium in the virgin alloy. Not only are the average values of life-to-rupture and percent prior creep substantially increased by means of the present invention, but also the virgin alloys of the present invention do not fail in second stage creep as sometimes happens with virgin heats ofAlloy B. 103,000

AS CZlSIElFr 5 Characteristic Treated* Treated* Y.S. (psi) Treated* Results of room temperature tests madeon specimens machined from cast turbine wheels of Alloy F and a cast modification thereof (Alloy 16) containing 1.5 percent hafni um in place of an equal percent by weight of nickel are set forth in Table X11.

Heat treatment comprises holding at 1800F. for 4 hours Table X11 shows that for castings of Alloy F, the present invention is effective not only to increase tensile elongation but also to provide a casting which is less sensitive to the deleterious effects of a heat treatment, which heat treatment is often necessary as a portion of process for providing on the casting a protective coating against corrosion. Additional data indicates that castings of the present invention exhibit highly advantageous fracture toughness characteristics.

While the present invention has been described in conjunction with advantageous embodiments, those skilled in the art will recognize that modifications and variations may be resorted to without departing from he spirit and scope of the invention. Such modifications and variations are considered to be within the purview and scope of the invention.

We claim:

1. A casting process comprising casting an alloy having a nickel base and containing chromium at least about 6.5 percent total aluminum plus titanium, less than about 6 percent tantalum and about 0.5 percent to about 5 percent by weight of hafnium and having in the solidified state a gamma matrix phase. a precipitate of gamma-prime and a eutectic gammaprime phase and a carbide phase dispersed in said matrix from the molten state into a mold having essentially the final con-- figuration of use of the object thus cast, and causing said alloy to solidify and cool in said mold at a rapid rate through the range of about 2,200 to 1,850 F. to thereby induce in said casting a high proportion of metal thermally conditioned to exhibit good mechanical characteristics at both high temperatures about l,800 F. and at intermediate temperatures about 1,400 F.

2. A process as in claim 1 wherein the casting is conducted under high vacuum.

3. A process as in claim 1 wherein thealloy in addition to hafnium contains in percent by weight about 0.2 percent to about 0.5 percent carbon, about 7 percent to about 15 percent chromium, up to about 35 percent cobalt, up to about 14 tungsten, up to about 8 percent molybdenum, about 0.5 percent to about 6 percent titanium, about 4 percent to about 7 percent aluminum, about 6.5 percent to about 10.5 percent aluminum plus titanium, up to about 0.3 percent boron, up to about 0.5 percent zirconium, up to about 3 percent columbium, up to about 1.5 percent vanadium, with the balance being essentially nickel in an amount of at least about 36 percent.

4. A process as in claim 3 wherein the alloy cast is an alloy selected from the group of alloys A to F modified to contain LS 4.25 L5 1.0 2.0 0.8! 4.7 0.6 5.5 6.0 5.0 6.1 5.5 5.9 I 1 0.015 0.015 0.015 0.012 0.014 0.01 and said alloy is cast under high vacuum. (H5 (H Q Q 5. A process as in claim 4 wherein the mold has the configu- L0 0 ration of a gas turbine structure. 88!. Ba]. Ba]. Bal. Hal. B 

2. A process as in claim 1 wherein the casting is conducted under high vacuum.
 3. A process as in claim 1 wherein the alloy in addition to hafnium contains in percent by weight about 0.2 percent to about 0.5 percent carbon, about 7 percent to about 15 percent chromium, up to about 35 percent cobalt, up to about 14 tungsten, up to about 8 percent molybdenum, about 0.5 percent to about 6 percent titanium, about 4 percent to about 7 percent aluminum, about 6.5 percent to about 10.5 percent aluminum plus titanium, up to about 0.3 percent boron, up to about 0.5 percent zirconium, up to about 3 percent columbium, up to about 1.5 percent vanadium, with the balance being essentially nickel in an amount of at least about 36 percent.
 4. A process as in claim 3 wherein the alloy cast is an alloy selected from the group of alloys A to F modified to contain about 0.5 percent to about 5 percent by weight of hafnium in place of an equal percent by weight of nickel Percent by Weight Alloy A B C D E F C 0.16 0.10 0.15 0.12 .18 0.05 Cr 9.0 8.0 9.0 12.5 10.0 12.0 Co 10.0 10.0 10.0 -15.0 -W 10.0 - 12.5 - - -Mo 2.5 6.0 - 4.2 3.0 4.5 Ta 1.5 4.25 - - - -Ti 1.5 1.0 2.0 0.8 t 4.7 0.6 Al 5.5 6.0 5.0 6.1 5.5 5.9 B 0.015 0.015 0.015 0.012 0.014 0.01 Zr 0.05 0.10 0.05 0.10 0.06 0.10 Cb - - 1.0 2.0 -2.0 V - - - - 1.0 -Ni Bal. Bal. Bal. Bal. Bal. Bal and said alloy is cast under high vacuum.
 5. A process as in claim 4 wherein the mold has the configuration of a gas turbine structure. 